Presently, optical recording systems enjoy great commercial appeal because of their high storage capacity, cost effectiveness, and reduced susceptibility to noise and data corruption. Optical recording systems are ideal for storing vast mounts of data on a permanent or long term basis.
Typically, optical recording systems use a laser or some other form of light source to "read" and "write" data from/to an optical medium. In a "write" operation, the light is directed by means of a lens assembly and focused onto the optical medium. The focused light causes the magnetic characteristic of a portion, commonly referred to as a "domain", of the optical medium to become altered. The optical medium retains the altered characteristic, even after the light source has been removed. By pulsing the laser to write domains, digital data can be "written" and stored onto the optical medium. Later, the data are retrieved by directing the light to the optical medium and reading the information indicated by the domains. The data are "read" from the medium by detecting the signal contained in the light reflected off the medium.
In the past, pulse position modulation (PPM) was used to record the digital data onto the optical medium. In PPM systems, the value of each instantaneous sample of a modulating signal is caused to modulate the position in time of a pulse. However, pulse width modulation (PWM) has recently been applied to optical recording systems because of its capability to increase storage capacity. In a PWM scheme, the value of each instantaneous sample of the modulating signal is caused to modulate the duration of a pulse. Hence, in PPM recording, each isolated magnetic domain signifies a single "1". Whereas, in PWM recording, a "1" is signified by a change from one magnetic state to another. Essentially, two "1's" are signified by the edges of a magnetic domain. Consequently, PWM effectively doubles the amount of information that can be recorded within a given storage area. Increasing the areal density is advantageous because it reduces cost and increases the mount of data that can be stored within a given area.
However, one problem with utilizing a PWM scheme is that a threshold must be established very precisely. Referring to FIG. 1, the positions of the "1" bits in PPM recording are determined by detecting the peaks of the resulting analog readback waveform. The peaks can be precisely determined by differentiating the analog waveform and detecting where the differentiated waveform crosses zero volts (e.g., zero crossings). In contrast, for PWM recording, a "1" bit is determined by the location where the analog signal crosses a pre-defined threshold level. The timing of these "1" bits is extremely dependent on the positioning of the threshold level. Even millivolt errors in the threshold level can produce nano-second variations of the data bits. Given that the widths of the domains are in micrometers, small errors in the threshold level can easily result in read errors.
Thus, there is a need in the prior art for an apparatus and method for determining and setting a threshold level in an optical recording system having a pulse width modulation scheme.